Posted April 13, 2022

Jason Gertz, Ph.D., University of Utah
Jennifer Richer, Ph.D., University of Colorado at Denver

Two-thirds of all primary breast cancers are estrogen receptor alpha (ER)-positive. Aromatase inhibitors (AIs), which suppress estrogen production, are the first line of treatment for the majority of ER-positive patients. However, virtually all breast cancers that recur following treatment are AI-resistant metastases with about 40% carrying mutations in the ER gene. With funding from a Fiscal Year 2015 (FY15) Breakthrough Award - Funding Level 1 - Partnering PI Option, Dr. Jason Gertz and Dr. Jennifer Richer sought to investigate the biological consequences of ER mutations and determine whether these mutations, or associated pathways, represent potential therapeutic targets for treatment of AI-resistant metastatic breast cancer.

In a recent pair of companion publications in Cancer Research, the laboratories of Drs. Gertz and Richer created models of ER mutant breast cancer and analyzed patient samples to identify molecular changes and therapeutic vulnerabilities caused by ER mutations. The models of ER mutations enabled the discovery of thousands of genes that mutant ER regulates in an unexpected manner. Using biopsies from patients with ER-positive metastatic breast cancer resistant to AI therapy (clinical trial NCT02953860, funded by an FY13 Clinical Translational Research Award - Partnering PI Option to Dr. Anthony Elias and Dr. Richer), the research team found that metastases with ER mutations expressed significantly higher levels of both ER and progesterone receptor (PR) proteins than those with wild-type ER. Interestingly, wild type and mutant ER metastases expressed similar androgen receptor (AR), which could indicate a role for AR in metastasis irrespective of ER mutational status.

To investigate the role of AR, the team utilized ER mutant breast cancer models and found that following long-term estrogen deprivation to mimic AI therapy (the situation under which ER mutations arise), AR was found to be markedly upregulated. To determine whether AR provided a metastatic advantage to ER mutant cells, the team grew ER mutant and wild type breast cancer cell lines in anchorage-independent conditions and found increased AR protein, particularly in the mutant ER cells. Moreover, the survival advantage of the mutant ER cells in anchorage independent conditions was inhibited by enzalutamide, an AR antagonist currently approved for treatment of prostate cancer and under clinical investigation in breast cancer. To determine whether mutant ER cells have a selective survival advantage and promote metastases in vivo, the team introduced mutant ER and wild type ER expressing breast cancer cells into ovariectomized (estrogen deprived) mice. Results showed that mice with mutant ER breast cancer cells had a significantly higher lung metastatic burden compared to those with wild type ER breast cancer cells.

The team also investigated Chitinase-3-like protein 1 (CHI3L1), an immunomodulatory secreted protein that is well known to be AR-regulated in prostate cancer where its levels in blood correlate with disease progression. The researchers found elevated CHI3L1 in mutant ER cells, particularly in the absence of estrogen (modeling AI therapy). What's more, the ER mutant cells lost metastatic capacity when secreted CHI3L1 function was blocked. These results identify a specific protein, in addition to AR, that could be targeted in mutant ER cells to decrease metastatic potential.

In studies to identify pathways altered in ER mutant breast cancers, the investigators found that innate immune, viral recognition, and type-1 interferon (IFN) pathways were preferentially upregulated in mutant ER expressing cell lines. The team determined that the IFN-induced transmembrane (IFITM) IFITM3, which has been reported to play a role in AI resistance, was upregulated in ER mutant cell lines as well as ER mutant biopsies from patients with metastatic breast cancer. Since mutant ER cell lines showed enhanced innate immune pathway activation, the team also examined patient tumor biopsies for differences in tumor-infiltrating immune cells. Metastatic biopsies with mutant ER had higher levels of pro-tumor T regulatory cells and macrophages. Moreover, macrophages expressing programmed cell death-ligand 1 (PD-L1), a checkpoint protein that suppresses the immune system, were significantly higher in ER mutant metastases. Thus, immune checkpoint inhibitors that block P D-L1 may represent therapeutics to explore in mutant ER metastatic breast cancer.

Through this FY15 BCRP Breakthrough Award, Dr. Gertz, Dr. Richer, and their research teams have discovered several important characteristics of mutant ER breast cancer that may offer therapeutic vulnerabilities. Their results suggest that acquired differences in AR, CHI3L1, and IFITM3 and macrophage PD-L1 expression in mutant ER breast cancers support survival and metastasis. Therefore, therapies targeting these proteins may represent new therapeutic approaches.

Publications:

Williams MM, Spoelstra NS, Arnesen S, et al. 2021. Steroid hormone receptor and infiltrating immune cell status reveals therapeutic vulnerabilities of ESR1-mutant breast cancer. Cancer Research Feb 1; 81(3):732-746. PMID: 33184106

Arnesen S, Blanchard Z, Williams MM, et al. 2021. Estrogen receptor alpha mutations in breast cancer cells cause gene expression changes through constant activity and secondary effects. Cancer Research Feb 1; 81(3):539-551. PMID: 33184109

Link:

Public and Technical Abstracts: Molecular Modeling of Estrogen Receptor alpha Mutated Breast Cancer to Guide New Therapeutic Strategies

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